The German patent application 10 2010 042 350.5 proposes a high-pressure control device for an injection system which has the aforedescribed features. In this case the control is effected by digitally switching the inlet and outlet valves with one actuator or two actuators. Such a pump with digitally switched inlet and outlet valves has various advantages. However, even in this embodiment an additional mechanical pressure relief valve (PRV) arranged in the high-pressure path downstream of the outlet valve is necessary, since in the event of the fault “Maximum delivery of pump”, for example as a result of a defective outlet valve, there must be a possibility of reducing the pressure on the high-pressure side, so that the system does not “burst” as a result of the pressure increases occurring in this case.
Such a mechanical pressure relief valve of this type is arranged, for example, in the high-pressure pump in parallel to the pump outlet valve and, depending on the concept selected, either effects a relief of pressure in the cylinder chamber (hydraulically locked in this case, since it is closed in the pumping phase of the pump) or shuts off gradually on the low-pressure side upstream of the pump inlet valve. This pressure relief valve is activated, for example, in the following situations: because of a mechanical or electrical fault the electrically switched outlet valve can no longer be activated. The high-pressure pump no longer delivers fuel to the high-pressure region. The outlet valve is forced shut because of the pressure present on the high-pressure side (in the rail). If the vehicle is in coasting mode (no injection) or is switched off, the pressure of the volume enclosed on the high-pressure side downstream of the pump outlet valve (high-pressure petrol injection valves have no leakage) will rise in dependence on the fuel temperature. If the vehicle is switched off, for example at a system pressure of 50 bar, and if, for example, the fuel temperature rises from 20° C. to 40° C. in the high-pressure region (inside the rail) after the engine has been switched off (because of the engine waste heat with the vehicle stationary), the system pressure rises, for example, from 50 bar to 250 bar. If no pressure reduction possibility were then present on the high-pressure side, this would necessarily lead to bursting of the system with such temperature increases, or would prevent a limp-home mode since the injectors no longer open above a given maximum pressure.
With the above-described injection system in which a normally-closed (that is, “currentlessly-closed”) outlet valve is used, a mechanical pressure relief valve therefore continues to be necessary.